This invention relates to devices employing a micro-sized thermoelectric power generating device.
Small electronic devices, such as those using integrated circuits, continue to play a major role in all aspects of society. Batteries have traditionally been used to power such devices.
The energy in a battery dissipates over time, requiring battery replacement or recharging.
Portable devices can be powered using thermoelectric (xe2x80x9cTExe2x80x9d) effects. The thermoelectric material generates power based on a thermal gradient. The thermal gradient for a wristwatch may be created by one side of the watch being exposed to the air (the cool side) and the other side being exposed to the wearer""s wrist (the hot side). Preferably, the device is in direct contact with the hot region. Alternatively, the device may be in contact with a thermally-conducting material which, in turn, is in direct contact with the hot region. Heat rejection from the cold side of the thermoelectric device could be to the ambient air or to any other suitably cooler medium.
The thermoelectric material may be placed in thermal contact with the first substrate. Here, xe2x80x9cthermal contactxe2x80x9d and xe2x80x9cthermally attachedxe2x80x9d can encompass any connection where heat easily flows from one material to another. This does not necessarily require that the materials be in direct contact. A metallization layer (described below) may be disposed between the substrate and the thermoelectric material to ensure that these materials are in thermal contact and mechanically attached.
In power-generation devices, a xe2x80x9cnaturalxe2x80x9d temperature gradient may be used to generate power through the Seebeck effect. In these devices, the thermoelectric materials may be sandwiched between a pair of thermally conducting electrically insulating substrates.
Diamond has the highest thermal conductivity (about 2400 Wmxe2x88x921Kxe2x88x921) of any known material. Diamond also has an extremely high degree of hardness, and is an excellent electrical insulator.
The present system describes a new kind of thermoelectric device that has advantages. Novel techniques of forming the system are also described.
An important advantage includes the ability to form a large number of thermoelectric legs due to the improved formation process.
A thermoelectric material, such as a BiTe alloy including Bi2Te3 and Bi2Te3-based alloys (in bulk or film form), may be used as the thermoelectric component. The thermoelectric material may be disposed between a first and second substrate. This material is patterned as a series of alternating xcexcm-sized n-type and p-type regions, or xe2x80x9clegsxe2x80x9d, on the substrates using microfabrication techniques. The area of the substrates may be larger than the area of the p and n leg region. The legs are preferably arranged in a two-dimensional xe2x80x9ccheckerboardxe2x80x9d pattern, and at least some are electrically in series so that the output voltage is the sum of the individual voltages. The devices are thermally in parallel. Of course, the voltage potential can be controlled by connecting some of the legs in parallel as well.
The thermoelectric material is preferably microfabricated according to techniques described herein.
The disclosed microfabrication techniques allow forming thousands of legs, e.g., 1000-20,000 legs. They also offer the potential to achieve an appropriate voltage/current combination for a given temperature differential across a device.
Films of BiTe alloys deposited as legs on the substrate typically have thicknesses of between 5-100 xcexcm. FIG. 2 shows the power output of a device as a function of leg thickness. The power output increases for a given cross-sectional area as the thermoelectric legs become thinner for a given temperature difference. This results in a higher output power density. Thin films of the thermoelectric material are therefore better if the thermal resistances or electrical contact resistances are low or negligible. Thin films also allow fabrication by IC fabrication technology.
Electrically insulating materials having high-thermal conductivities, such as silicon carbide, aluminum nitride, boron nitride, or beryllium oxide, may be used in place of the diamond substrates. Other materials with similar electrically insulating and thermally conducting properties (i.e., as close to diamond as possible) could also be used. The desirable properties of diamond and materials having similar properties enhance the effectiveness of the device. During operation, input heat from the hot side is rapidly and evenly spread out so that the substrate efficiently supplies heat to all the n and p legs.
A multi-layer stack structure is preferably used to attach the thermoelectric material to the substrate. The stack structure preferably has electrically and thermally conductive materials. Electrically conductive materials provide a series electrical connection between the p- and n-doped legs of the thermoelectric material. A low electrical contact resistance between the electrically conductive materials and the thermoelectric legs is desirable. This reduces the total internal electrical resistance of the device and reduces performance degradation.
Thermally-conductive materials within the stack structure facilitate heat flow between the thermoelectric material and the substrate. A low thermal resistance between the heat-dissipating device and the thermoelectric material reduces heat losses. These combined factors prevent a degradation in the performance of the device. A lower stack structure having a similar multi-layer configuration (and similar electrical and thermal properties) connects the thermoelectric material to the second heat-conducting substrate.
A preferred multi-layer stack structure includes a metallization layer coated on the inner surface of the substrate. This thin metal coating facilitates adhesion of the substrate to other materials. In preferred embodiments, metals such as titanium or chromium are used as the substrate metallization layers. An outer diffusion barrier layer, preferably composed of ternary alloys of metal-Sixe2x80x94N, where the metal is a transition metal such as Ti or Ta, may then be deposited on the metallization layer.
The outer diffusion barrier layer prevents the diffusion of copper to the metallization layer and to the substrate. Depending on temperature, the outer diffusion barrier may not be required. For example, at room temperature, the outer diffusion barrier may not be needed to prevent more interdiffusion.
A copper layer is deposited on the outer diffusion barrier layer. An inner diffusion barrier layer, preferably composed of Pt or metal-Sixe2x80x94N, is then deposited on the copper layer. The inner diffusion barrier layer impedes the diffusion of copper (which has a high solid-state solubility and thus diffuses rapidly) into either the metallization layers or the thermoelectric material. Impeding the diffusion of copper prevents contamination of the other materials in the stack structure. An electrical contact layer, preferably including one of the transition metals, may be deposited if required on the inner diffusion barrier layer to complete the multi-layer upper stack structure.
P- and n-doped thermoelectric legs of the desired thickness are deposited on the electrical contact layer. A second electrical contact layer, followed by a second inner diffusion barrier layer, is deposited on the legs.
Each layer of the stack structures is preferably deposited using semiconductor device fabrication techniques.
Prior processes attempted to produce sharp, patterned thick photoresist structures using microelectrical mechanical system (xe2x80x9cMEMSxe2x80x9d) technology, which is often based on x-ray lithography.
The major factors which limit the power output of the device include: 1) the temperature differential across the thermoelectric legs, which is a function of the series electrical resistance of the thermoelectric legs; 2) the electrical contact resistance provided by the upper and lower multi-layer connecting structures; 3) the geometry and number of legs; and 4) the thermal resistances for heat transfer at the hot and cold surfaces of the legs.
An increase in the available temperature differential will increase the available energy.
The contact resistances are particularly important with respect to thermoelectric legs of short lengths. For example, the conversion efficiency of a real system is about 20% lower than the value calculated in FIG. 3 for the thermoelectric materials only. This figure shows dimensionless figure of merit which represents the performance of a thermoelectric device which depends solely upon the properties of the thermoelectric material and the hot side and cold side temperatures. ZT is proportional to the square of the Seebeck coefficient divided by the product of the electrical resistivity and the thermal conductivity. The best ZT values are obtained in heavily doped semiconductors, such as BiTe alloys, PbTe alloys and Sixe2x80x94Ge alloys. The electrical contact resistance arises from the connection of all the legs in series. Typical values obtained for actual generators and coolers are 10 to 25 xcexcxcexa9/cm2. The thermal contact resistance is generated by the heat transfer characteristics of the ceramic plates and contact layers used to build the thermoelectric module. The heat exchangers and corresponding heat losses should also be taken into account.
In addition, the transport properties of the thermoelectric materials vary with temperature, as illustrated in FIG. 1. When a thermoelectric device is operating across a wide temperature range, these variations may be factored in the calculation of its performance.
The smaller size coupled with integrated circuit technology allows the devices to handle higher power densities for both cooling and generator applications. In addition, thermoelectric device miniaturization enables its operation as a power generator at much higher voltages than is possible for bulk devices, due to the much larger number of thermoelectric elements (which may be two orders of magnitude larger). These higher voltages (1-100 V) are more compatible with other electronic components.
While the embodiment discloses a specific use, the teachings given herein may advantageously be used in many varieties of power generation devices. The invention may advantageously employ recovery of any kind of heat, e.g., waste heat.